The invention relates in general to a low dark current long wavelength metal-semiconductor-metal photodetector and in particular to a low dark current indium aluminum arsenide/indium gallium arsenide (InAlAs/InGaAs) interdigitated metal-semiconductor-metal (MSM) photodetector.
Metal-semiconductor-metal interdigitated photodetectors employing ternary III-V structures are well known in the art. Such structures often use InAlAs and InGaAs as detector and buffer material. One of the problems with such systems is that it is important to provide high speed and high sensitivity along with a good signal-to-noise-ratio. The signal-to-noise-ratio is degraded by noise contributed via such factors as background and dark current.
In a photodetector electrodes on a surface are biased to sweep out charge carriers produced by photon absorption. Dark current occurs at the electrodes by various thermionic and tunneling emission effects which cause current to flow in the III-V ternary photodetector to other electrodes. Unfortunately, however, the low efficiency of prior art devices, in part caused by the relatively high dark currents, is undesirable and leads to low signal noise ratios.
There have been attempts to improve the signal-to-noise-ratio of the devices by improving the amount of light which reaches the active region. The active or photoresponsive regions of the metal-semiconductor-metal devices are defined either by a mesa or by a 200 nanometer thick silicon nitride film positioned in the contact layer. For instance, in Seo, J. W. et al., "A Comparative Study of Metal-Semiconductor Metal-Photodetectors on GaAs with Indium-Tin-Oxide and Ti/Au Electrodes," IEEE Photonics Technology Letters, Vol. 4, No. 8, Aug. 1992; devices are disclosed including indium tin oxide as a transparent electrode material having a relatively high electrical conductivity and low optical absorption coefficient. Those devices, however, exhibit a relatively high dark current. While they provide some improvement in the signal-to-noise-ratio, they do not provide significant reduction of the dark current. Other attempts have been made to use transparent electrodes to improve efficiency, for instance, in "Improved InP/InGaAs/InP PIN Detector Response Using a Transparent Conductor Contact," Electronics Letters, Aug. 2, 1990, Volume 26, No. 16.
Thus, what is needed is an improved low dark current photodetector for providing an enhanced signal-to-noise-ratio.